The present invention relates to improvements in the mounting of electronic devices and the like in relation to their associated heat sinks, and, in one particular aspect, to novel and improved thin readily-applied mounting pad arrangements of low-cost composite construction which establish high-quality thermal transfer characteristics between semiconductor devices and heat sinks largely by way of crushable expanded-graphite particle material.
As is well known, at the current stage of developments in the art, applications of miniature semiconductor devices and microcircuit modules are severely limited in respect of the self-generated heat which they can successfully expel and prevent from building up as they are caused to operate at higher and higher powers. In some instances, the environments of use permit complex forced-fluid cooling systems to be employed, but a very common and economical expedient instead involves the attachment of simpler finned metal extrusions or stampings which help to conduct and radiate heat away from the thermally-vulnerable regions. For such purposes, it is important that the thermal impedance between a semiconductor or microcircuit unit and its associated heat sink structure be kept to a minimum and that it be of uniformity which will prevent build-up of localized hot spots. Those characteristics are not always realized to a satisfactory extent by simply abutting some part of the heat-generating unit with complementary surfaces of its heat sink, because, despite appearances, the respective mating surfaces will generally exhibit only a very surprisingly small percentage of area in actual sound contact. In part, such contact failure, and attendant poor transmission of heat, tends to be caused by relatively gross surface imperfections, whose effects might be offset somewhat by subjecting the complementary surfaces to precise machining or other shaping. And, in part, such contact failures can be attributed to microscopic surface irregularities, which will remain at the interfaces even if the cooperating parts are formed and finished with great care. When it becomes necessary to insulate one part from the other, electrically, the heat-conduction problems are greatly compounded.
Among the prior practices which have been employed in efforts to improve the heat flow from semiconductor or like devices to their heat sinks is that of spreading amorphous oil or grease, such as silicone, between the joined surfaces. The messy character of such a filler, as well as the uses of insulating mica and varnish insulating layers, are referred to in U.S. Pat. No. 3,229,757. Thermal grease and/or powdered metal, contained by a film, has likewise been proposed to augment heat transfer, in U.S. Pat. No. 4,092,697, although the fabrication and handling of such small "pillows" obviously involves special problems also. Cooling fins have been secured by way of epoxy cement loaded with powdered metal (U.S. Pat. No. 3,261,396), and large-area epoxy films have been said to insulate while yet transferring large quantities of heat (U.S. Pat. No. 3,611,046). In U.S. Pat. No. Re. 25,184, electrically non-conductive plastic coating material is filled with molybdenum disulphide to promote heat conduction, and, where electrical insulation is not essential, a dimpled malleable metal wafer has been interposed to increase transfer of heat (U.S. Pat. No. 4,151,547).